1. Field of the Invention
The present invention relates to a method of forming a magnetic memory, in particularly, a method of forming a magnetic random access memory that stores data in a nonvolatile manner by utilizing spontaneous magnetization of a metallic ferromagnetic substance.
2. Related Background Art
Magnetic (magnetoresistive) random access memories (hereinafter referred to as the “MRAMs”) are under development as one type of semiconductor memories that store data in a nonvolatile manner. In the MRAMS, the direction of spontaneous magnetization of the ferromagnetic film is associated with “1” or “0”, which represents a digital data.
Data stored in a MRAM is read by utilizing a magneto-resistance effect that the ferromagnetic substance exhibits. The magneto-resistance effect has two types. One of which is a giant magneto-resistance effect (GMR) and the other is a tunnel magneto-resistance effect (TMR). In the following description, memory cells that use the GMR are referred to as GMR cells and memory cells that use the TMR are referred to as TMR cells.
It is required to process a ferromagnetic film in order to form a memory cell of an MRAM. Under present circumstances, it is difficult to process the ferromagnetic film through chemical dry etching. Therefore, in general, the ferromagnetic film is patterned by ion milling.
FIGS. 3A to 3I show a method of forming a TMR cell of a related art.
As shown in FIG. 3A, a silicon oxide film 102, an aluminum film 103, a first magnetic film 104, an insulating film 105, and a second magnetic film 106 are formed in succession on a substrate 101. As shown in FIG. 3B, a resist pattern 107 is formed on the second ferromagnetic film 106. Then, the second magnetic film 106, the insulating film 105, the first magnetic film 104, and the aluminum film 103 are etched in succession by ion milling using the resist pattern 107 as a mask. As a result of this etching, as shown in FIG. 3C, there are formed a lower electrode 103′ and a fixed magnetization (a pinned magnetic) layer 104′ of the TMR cell. Further, the resist pattern 107 is removed by ashing in O2 plasma.
As shown in FIG. 3D, a resist pattern 108 is formed on the second magnetic film 106. Then, the second magnetic film 106 and the insulating film 105 are etched by ion milling using the resist pattern 108 as a mask. As a result, as shown in FIG. 3E, there are formed an insulating layer 105′ and a free magnetization (free magnetic) layer 106′ of the TMR cell. Further, the resist pattern 108 is removed by ashing in O2 plasma. As shown in FIG. 3F, there is formed a silicon oxide film 109, which is an insulating film, on the whole upper surface of the substrate 101.
As shown in FIG. 3G, a resist pattern 110 is formed to form a contact hole. As shown in FIG. 3H, the silicon oxide film 109 is etched using the resist pattern, 110 as a mask, thereby forming a contact hole 111 reaching the free magnetization layer 106′. As shown in FIG. 3I, there is formed a wiring layer 112, which is electrically connected to the free magnetization layer 106′, using a conductive material, such as aluminum. In this manner, a TMR cell is formed.
However, such a method arises problems described below. FIG. 4 illustrates a memory cell of a related art to explain the problems. As the first problem, oxide layers 104a′ and an oxide layer 106a′ are formed on the surfaces of the fixed magnetization layer 104′ and the free magnetization layer 106′, respectively. As shown in FIG. 3C, the surface of the second ferromagnetic film 106 is exposed to O2 plasma during the removal of the resist pattern 107. Accordingly, the surface of the second ferromagnetic film 106 is oxidized and the oxide layer 106a′ is formed on the surface of the free magnetization layer 106′. Similarly, as shown in FIG. 3E, the surface of the fixed magnetization layer 104′ is exposed to O2 plasma during the removal of the resist pattern 108. Accordingly, the oxide layers 104a′ are formed on the surface of the fixed magnetization layer 104′.
The stated oxidization of the surfaces of the fixed magnetization layer 104′ and the free magnetization layer 106′ leads to the degradation of characteristics of the TMR cell. Therefore, such oxidization is not preferable.
As the second problem, as shown in FIG. 4, side walls 113 and side walls 114 protruding perpendicular to the substrate 101 are formed on the side surfaces of the free magnetization layer 106′ and the fixed magnetization layer 104′. The sidewalls 113 existing on the side surfaces of the fixed magnetization layer 104′ are formed during the etching by ion milling shown in FIGS. 3B and 3C. During the etching by ion milling, materials forming the second ferromagnetic film 106, the insulating film 105, the first ferromagnetic film 104, and the aluminum film 103 are sputtered. As a result, the materials adhere to the side surfaces of the resist pattern 107. The adherents are not removed but are left even if the resist pattern 107 is removed by ashing. As a result, the side walls 113 are formed by the adherents. Similarly, the side walls 114 existing on the side surfaces of the free magnetization layer 106′ are formed during the etching by ion milling shown in FIGS. 3D and 3E. During this etching, materials forming the second ferromagnetic film 106 and the insulating film 105 are sputtered. As a result, the materials adhere to the side surfaces of the resist pattern 108. The adherents are not removed but are left even if the resist pattern 108 is removed by ashing. As a result, the side walls 114 are formed by the adherents. The height of each of the side walls 113 and the side walls 114 is about the thickness of one of the resist patterns 107 and 108, typically about 1 μm. The side walls 113 and the side walls 114 having heights of about 1 μm are unstable and tend to topple over.
Such shapes of the side walls 113 and the side walls 114 lead to defects in the shape of an MRAM and therefore is not preferable. The stated shapes of the side walls 113 and the side walls 114 impair the coverage property of the interlayer insulating film 109. Further, if the side walls 113 and the side walls 114 standing upright topple over, the shape of the interlayer insulating film 109 becomes abnormal. These cause wire breaking and a short circuit of the MRAM and lead to the malfunction of the MRAM.
It is desired that there is provided a technique with which a memory cell of an MRAM is formed while preventing the oxidation of a ferromagnetic film included in the memory cell.
Further, it is desired that there is provided a technique of manufacturing an MRAM in which no malfunction of the MRAM is caused by side walls that have been formed on the side surfaces of a mask during the processing of a ferromagnetic film by ion milling.
It should be noted here that as a technique that may have a relation to the invention disclosed in this patent application, a technique of processing a magnetic substance is disclosed in Japanese Patent Application Laid-open No. 2000-339622. With this publicly known processing technique, a non-magnetic layer is made of alumina on the upper surface of a magnetic film. The magnetic film is etched by ion milling using this non-magnetic layer as a mask.
However, the Japanese Patent Application Laid-open No. 2000-339622 does not disclose the stated problem that the surface of a metallic ferromagnetic substance is oxidized. This publicly known processing technique is a method of forming a magnetic pole of a thin film magnetic head. The magnetic film of the thin film magnetic head is extremely thicker than the ferromagnetic film used in an MRAM. Therefore, the oxidation of the surface of the magnetic film does not become a considerable problem in the thin-film magnetic head. On the other hand, in a memory cell of an MRAM made of a ultra-thin metallic ferromagnetic substance, the oxidation of the surface of the ferromagnetic film may become a problem that influences the reliability of the memory.